Despeckling Apparatus and Method

ABSTRACT

An apparatus and method that reduces laser speckle by using stimulated Raman scattering in an optical fiber. The fiber core diameter and length are selected to achieve a desired output color. An adjustable despeckler is formed by combining two optical fibers in parallel and adjusting the amount of light in each path.

RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119(e) to U.S.provisional patent application, U.S. Ser. No. 61/267429, filed Dec. 7,2009, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

There are many advantages for using laser light sources to illuminatedigital projection systems, but the high coherence of laser light tendsto produce undesirable speckle in the viewed image. Known despecklingmethods generally fall into the categories of polarization diversity,angle diversion, and wavelength diversity. In the laser projectionindustry, there has been a long-felt need for more effective despecklingmethods.

SUMMARY OF THE INVENTION

In general, in one aspect, an optical apparatus where stimulated Ramanscattering in an optical fiber reduces the speckle of the light outputfrom the optical fiber.

Implementations may include one or more of the following features. Theoptical fiber core diameter and length may be selected to achieve adesired amount of reduction in speckle. The optical fiber may be amultimode fiber. The core diameter and length may be selected to achievea desired color of light output. The desired color may be a primarycolor. The primary color may be green. There may be an opticalparametric amplifier that generates blue and red. A portion of thestimulated Raman scattering may be red. There may be a red light sourcethat is combined with the red stimulated Raman scattering light. Aportion of the stimulated Raman scattering may be yellow and may be usedto form a projected digital image with four primary colors. The greenlight output may have a color chromaticity of u′=0.099+/−0.01 andv′=0.578+/−0.01. There may be a laser light source and a digitalprojector. The laser light source may illuminate the optical fiber, andthe optical fiber may illuminate the digital projector. The laser lightsource may include a pulsed laser and a second harmonic generator. Thelaser light source may have an output at 523.5 nm, 515 nm, or 532 nm.There may be a light coupling system and a homogenizing device. Thelaser light source may illuminate the light coupling system, the lightcoupling system may illuminate the optical fiber, the optical fiber mayilluminate the homogenizing device, and the homogenizing device mayilluminate the digital projector. There may be a second optical fiberand a variable light splitter. The variable light splitter mayilluminate the first optical fiber and the second optical fiber, andstimulated Raman scattering in the second optical fiber may reducespeckle from the second optical fiber. The variable light splitter mayinclude a rotating waveplate and a polarizing beam splitter. The secondoptical fiber may have a core diameter and length selected to achieve adesired amount of reduction in speckle. The laser light source mayilluminate the variable light splitter, and the first optical fiber andthe second optical fiber may illuminate the digital projector. Thevariable light splitter may be adjusted to achieve the desired color fora combined light output of the first optical fiber and the secondoptical fiber.

In general, in one aspect, a laser projector including a blue laserlight source, an optical fiber, and a spatial light modulator. The bluelaser light source illuminates the optical fiber, the optical fiberilluminates the spatial light modulator, and stimulated Raman scatteringin the optical fiber produces despeckled green light and despeckled redlight.

In general, in one aspect, a method of despeckling that includesgenerating a laser beam, focusing the laser beam into an optical fiber,generating stimulated Raman scattering light in the optical fiber, andusing the stimulated Raman scattering light to form a projected digitalimage

In general, in one aspect, a method of despeckling that includesgenerating a first laser beam, splitting the first laser beam intosecond and third laser beams, focusing the second laser beam into afirst optical fiber, generating a first stimulated Raman scatteringlight in the first optical fiber, focusing the third laser beam into asecond optical fiber, generating a second stimulated Raman scatteringlight in the second optical fiber, combining the first stimulated Ramanscattering light and the second stimulated Raman scattering light toform a combined stimulated Raman scattering light, using the combinedstimulated Raman scattering light to form a projected digital image, andadjusting the amount of light in the second laser beam and the thirdlaser beam to achieve a desired color for a primary color in theprojected digital image.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a graph of stimulated Raman scattering at moderate power;

FIG. 2 is a graph of stimulated Raman scattering at high power;

FIG. 3 is a top view of a laser projection system with a despecklingapparatus;

FIG. 4 is a color chart of a laser-projector color gamut compared to theDigital Cinema Initiative (DCI) and Rec. 709 standards;

FIG. 5 is a graph of color vs. power for a despeckling apparatus;

FIG. 6 is a graph of speckle contrast and luminous efficacy vs. colorfor a despeckling apparatus;

FIG. 7 is a top view of a laser projection system with an adjustabledespeckling apparatus;

FIG. 8 is a graph of percent power into the first fiber, color out ofthe first fiber, and color out of the second fiber vs. total power foran adjustable despeckling apparatus;

FIG. 9 is a top view of a three-color laser projection system with anadjustable despeckling apparatus;

FIG. 10 is a block diagram of a three-color laser projection system withdespeckling of light taken after an OPO;

FIG. 11 is a block diagram of a three-color laser projection system withdespeckling of light taken before an OPO;

FIG. 12 is a block diagram of a three-color laser projection system withdespeckling of light taken before and after an OPO;

FIG. 13 is a flowchart of a despeckling method; and

FIG. 14 is a flowchart of an adjustable despeckling method.

DETAILED DESCRIPTION

Raman gas cells using stimulated Raman scattering (SRS) have been usedto despeckle light for the projection of images as described in U.S.Pat. No. 5,274,494. SRS is a non-linear optical effect where photons arescattered by molecules to become lower frequency photons. A thoroughexplanation of SRS is found in Nonlinear Fiber Optics by Govind Agrawal,Academic Press, Third Edition, pages 298-354. FIG. 1 shows a graph ofstimulated Raman scattering output from an optical fiber at a moderatepower which is only slightly above the threshold to produce SRS. Thex-axis represents wavelength in nanometers (nm) and the y-axisrepresents intensity on a logarithmic scale in dBm normalized to thehighest peak. First peak 100 at 523.5 nm is light which is not Ramanscattered. The spectral bandwidth of first peak 100 is approximately 0.1nm although the resolution of the spectral measurement is 1 nm, so thewidth of first peak 100 cannot be resolved in FIG. 1. Second peak 102 at536.5 nm is a peak shifted by SRS. Note the lower intensity of secondpeak 102 as compared to first peak 100. Second peak 102 also has a muchlarger bandwidth than first peak 100. The full-width half-maximum (FWHM)bandwidth of second peak 102 is approximately 2 nm as measured at pointswhich are −3 dBm down from the maximum value. This represents a spectralbroadening of approximately 20 times compared to first peak 100. Thirdpeak 104 at 550 nm is still lower intensity than second peak 102. Peaksbeyond third peak 104 are not seen at this level of power.

Nonlinear phenomenon in optical fibers can include self-phasemodulation, stimulated Brillouin Scattering (SBS), four wave mixing, andSRS. The prediction of which nonlinear effects occur in a specific fiberwith a specific laser is complicated and not amenable to mathematicalmodeling, especially for multimode fibers. SBS is usually predicted tostart at a much lower threshold than SRS and significant SBS reflectionwill prevent the formation of SRS. One possible mechanism that can allowSRS to dominate rather than other nonlinear effects, is that the modestructure of a pulsed laser may form a large number closely-spaced peakswhere each peak does not have enough optical power to cause SBS.

FIG. 2 shows a graph of stimulated Raman scattering at higher power thanin FIG. 1. The x-axis represents wavelength in nanometers and the y-axisrepresents intensity on a logarithmic scale in dBm normalized to thehighest peak. First peak 200 at 523.5 nm is light which is not Ramanscattered. Second peak 202 at 536.5 nm is a peak shifted by SRS. Notethe lower intensity of second peak 202 as compared to first peak 200.Third peak 204 at 550 nm is still lower intensity than second peak 202.Fourth peak 206 at 564 nm is lower than third peak 204, and fifth peak208 at 578 nm is lower than fourth peak 206. At the higher power of FIG.2, more power is shifted into the SRS peaks than in the moderate powerof FIG. 1. In general, as more power is put into the first peak, moreSRS peaks will appear and more power will be shifted into the SRS peaks.In the example of FIGS. 1 and 2, the spacing between the SRS peaks isapproximately 13 to 14 nm. As can be seen in FIGS. 1 and 2, SRS produceslight over continuous bands of wavelengths which are capable ofdespeckling by the mechanism of wavelength diversity. Strong despecklingcan occur to the point where the output from an optical fiber with SRSshows no visible speckle under most viewing circumstances. Maximum andminimum points for speckle patterns are a function of wavelength, soaveraging over more wavelengths reduces speckle. A detailed descriptionof speckle reduction methods can be found in Speckle Phenomena inOptics, by Joseph W. Goodman, Roberts and Company Publishers, 2007,pages 141-186.

FIG. 3 shows a top view of a laser projection system with a despecklingapparatus based on SRS in an optical fiber. Laser light source 302illuminates light coupling system 304. Light coupling system 304illuminates optical fiber 306 which has core 308. Optical fiber 306illuminates homogenizing device 310. Homogenizing device 310 illuminatesdigital projector 312. Illuminating means making, passing, or guidinglight so that the part which is illuminated utilizes light from the partwhich illuminates. There may be additional elements not shown in FIG. 3which are between the parts illuminating and the parts beingilluminated. Light coupling system 304 and optical fiber 306 with core308 form despeckling apparatus 300. Laser light source 302 may be apulsed laser that has high enough peak power to produce SRS in opticalfiber 306. Light coupling system 304 may be one lens, a sequence oflenses, or other optical components designed to focus light into core308. Optical fiber 306 may be an optical fiber with a core size andlength selected to produce the desired amount of SRS. Homogenizingdevice 310 may be a mixing rod, fly's eye lens, diffuser, or otheroptical component that improves the spatial uniformity of the lightbeam. Digital projector 312 may be a projector based on digitalmicromirror (DMD), liquid crystal device (LCD), liquid crystal onsilicon (LCOS), or other digital light valves. Additional elements maybe included to further guide or despeckle the light such as additionallenses, diffusers, vibrators, or optical fibers.

For standard fused-silica fiber with a numerical aperture of 0.22, thecore size may be 40 micrometers diameter and the length may be 110meters when the average input power is 3 watts at 523.5 nm. For higheror lower input powers, the length and/or core size may be adjustedappropriately. For example, at higher power, the core size may beincreased or the length may be decreased to produce the same amount ofSRS as in the 3 watt example. FIG. 1 shows the spectral output of astandard fused-silica fiber with a numerical aperture of 0.22, core sizeof 40 micrometers diameter and length of 110 meters when the averageinput power is 2 watts at 523.5 nm. FIG. 2 shows the output of the samesystem when the average input power is 4 watts. In both cases, thepulsed laser is a Q-switched, frequency-doubled neodymium-doped yttriumlithium fluoride (Nd:YLF) laser which is coupled into the optical fiberwith a single aspheric lens that has a focal length of 18.4 mm.Alternatively, a frequency-doubled neodymium-doped yttrium aluminumgarnet (Nd:YAG) laser may be used which has an optical output wavelengthof 532 nm. The examples of average input powers in this specificationare referenced to laser pulses with a pulse width of 50 ns and afrequency of 16.7 kHz.

FIG. 4 shows a color chart of a laser-projector color gamut compared tothe DCI and Rec. 709 standards. The x and y axes of FIG. 4 represent theu′ and v′ coordinates of the Commission Internationale de l'Eclairage(CIE) 1976 color space. Each color gamut is shown as a triangle formedby red, green, and blue primary colors that form the corners of thetriangle. Other colors of a digital projector are made by mixing variousamounts of the three primaries to form the colors inside the gamuttriangle. First triangle 400 shows the color gamut of a laser projectorwith primary colors at 452 nm, 523.5 nm, and 621 nm. Second triangle 402shows the color gamut of the DCI standard which is commonly accepted fordigital cinema in large venues such as movie theaters. Third triangle404 shows the color gamut of The International Telecommunication UnionRadiocommunication (ITU-R) Recommendation 709 (Rec. 709) standard whichis commonly accepted for broadcast of high-definition television. Greenpoint 410 is the green primary of a laser projector at 523.5 nm. Redpoint 412 is the red primary of a laser projector at 621 nm. Line 414(shown in bold) represents the possible range of colors along thecontinuum between green point 410 and red point 412. The colors alongline 414 can be are obtained by mixing yellow, orange, and red colorswith the primary green color. The more yellow, orange, or red color, themore the color of the green is pulled along line 414 towards the reddirection. For the purposes of this specification, “GR color” is definedto be the position along line 414 in percent. For example, pure green atgreen point 410 has a GR (green-red) color of 0%. Pure red at red point412 has a GR color of 100%. DCI green point 416 is at u′=0.099 andv′=0.578 and has a GR color of 13.4% which means that the distancebetween green point 410 and DCI green point 416 is 13.4% of the distancebetween green point 410 and red point 412. When the Rec. 709 green pointof third triangle 404 is extrapolated to line 414, the resultant Rec.709 green point 418 has a GR color of 18.1%. The concept of GR color isa way to reduce two-dimensional u′ v′ color as shown in thetwo-dimensional graph of FIG. 4 to one-dimensional color along line 414so that other variables can be easily plotted in two dimensions as afunction of GR color. In the case of a primary green at 523.5 nmexperiencing SRS, the original green color is partially converted toyellow, orange, and red colors, which pull the resultant combinationcolor along line 414 and increase the GR %. Although the DCI green pointmay be the desired target for the green primary, some variation in thecolor may be allowable. For example, a variation of approximately+/−0.01 in the u′ and v′ values may be acceptable.

FIG. 5 shows a graph of color vs. power for a despeckling apparatus. Thex-axis represents power in watts which is output from the optical fiberof a despeckling apparatus such as the one shown in FIG. 3. The y-axisrepresents the GR color in percent as explained in FIG. 4. The opticalfiber has the same parameters as in the previous example (core diameterof 40 micrometers and length of 110 meters). Curve 500 shows how thecolor varies as a function of the output power. As the output powerincreases, the GR color gradually increases. The curve can be fit by thethird-order polynomial

GR % =1.11p ³+0.0787p ²+1.71p+0.0041

where “p” is the output power in watts. First line 502 represents theDCI green point at a GR color of 13.4%, and second line 504 representsthe Rec. 709 green point at approximately 18.1%. The average poweroutput required to reach the DCI green point is approximately 2.1 W, andthe average output power required to reach the Rec. 709 point isapproximately 2.3 W.

FIG. 6 shows a graph of speckle contrast and luminous efficacy vs. colorfor a despeckling apparatus such as the one shown in FIG. 3. The x-axisrepresents GR color in percent. The left y-axis represents specklecontrast in percent, and the right y-axis represents luminous efficacyin lumens per watt. Speckle contrast is a speckle characteristic thatquantitatively represents the amount of speckle in an observed image.Speckle contrast is defined as the standard deviation of pixelintensities divided by the mean of pixel intensities for a specificimage. Intensity variations due to other factors such as non-uniformillumination or dark lines between pixels (screen door effect) must beeliminated so that only speckle is producing the differences in pixelintensities. Measured speckle contrast is also dependent on themeasurement geometry and equipment, so these should be standardized whencomparing measurements. Other speckle characteristics may bemathematically defined in order to represent other features of speckle.In the example of FIG. 6, the measurement of speckle contrast wasperformed by analyzing the pixel intensities of images taken with aCanon EOS Digital Rebel XTi camera at distance of two screen heights.Automatic shutter speed was used and the iris was fixed at a 3 mmdiameter by using a lens focal length of 30 mm and an f# of 9.0.Additional measurement parameters included an ISO of 100, monochromedata recording, and manual focus. The projector was a Digital ProjectionTitan that was illuminated with green laser light from a Q-switched,frequency-doubled, Nd:YLF laser which is coupled into a 40-micrometercore, 110 meter, optical fiber with a single aspheric lens that has afocal length of 18.4 mm. Improved uniformity and a small amount ofdespeckling was provided by a rotating diffuser at the input to theprojector.

For the speckle-contrast measurement parameters described above, 1%speckle is almost invisible to the un-trained observer with normalvisual acuity when viewing a 100% full-intensity test pattern.Conventional low-gain screens have sparkle or other non-uniformitiesthat can be in the range of 0.1% to 1% when viewed with non-laserprojectors. For the purposes of this specification, 1% speckle contrastis taken to be the point where no speckle is observable for mostobservers under most viewing conditions. 5% speckle contrast is usuallyquite noticeable to un-trained observes in still images, but is oftennot visible in moving images.

First curve 600 in FIG. 6 shows the relationship between measuredspeckle contrast and GR color. As the GR color is increased, the specklecontrast is decreased. Excellent despeckling can be obtained such thatthe speckle contrast is driven down to the region of no visible specklenear 1%. In the example of FIG. 6, first line 602 represents the DCIgreen point which has a speckle contrast of approximately 2% and secondline 604 represents the Rec. 709 green point which has a specklecontrast of approximately 1%. The speckle contrast obtained in aspecific configuration will be a function of many variables includingthe projector type, laser type, fiber type, diffuser type, andspeckle-contrast measurement equipment. Third line 606 represents theminimum measurable speckle contrast for the system. The minimummeasurable speckle contrast was determined by illuminating the screenwith a broadband white light source and is equal to approximately 0.3%in this example. The minimum measurable speckle contrast is generallydetermined by factors such as screen non-uniformities (i.e. sparkle) andcamera limitations (i.e. noise).

Second curve 608 in FIG. 6 shows the relationship between white-balancedluminous efficacy and GR color. The white-balanced luminous efficacy canbe calculated from the spectral response of the human eye and includesthe correct amounts of red light at 621 nm and blue light at 452 nm toreach the D63 white point. As the GR color is increased in the rangecovered by FIGS. 6 (0% to 25%) the white-balanced luminous efficacyincreases almost linearly from approximately 315 lm/w at a GR color of0% to approximately 370 lm/w at the DCI green and approximately 385 lm/wat the Rec. 709 green point. This increase in luminous efficacy isbeneficial to improve the visible brightness and helps compensate forlosses that are incurred by adding the despeckling apparatus.

FIG. 7 shows a top view of a laser projection system with an adjustabledespeckling apparatus. FIG. 7 incorporates two fibers for despecklingrather than the one fiber used for despeckling in FIG. 3. Thedespeckling apparatus of FIG. 3 allows tuning of the desired amount ofdespeckling and color point by varying the optical power coupled intooptical fiber 306. FIG. 7 introduces a new independent variable which isthe fraction of optical power coupled into one of the fibers. Thebalance of the power is coupled into the other fiber. The total powersent through the despeckling apparatus is the sum of the power in eachfiber. The additional variable allows the despeckling and color point tobe tuned to a single desired operation point for any optical power overa limited range of adjustment.

In FIG. 7, polarized laser light source 702 illuminates rotatingwaveplate 704. Rotating waveplate 704 changes the polarization vector ofthe light so that it contains a desired amount of light in each of twopolarization states. Rotating waveplate 704 illuminates polarizingbeamsplitter (PBS) 706. PBS 706 divides the light into two beams. Onebeam with one polarization state illuminates first light coupling system708. The other beam with the orthogonal polarization state reflects offfold mirror 714 and illuminates second light coupling system 716. Firstlight coupling system 708 illuminates first optical fiber 710 which hasfirst core 712. First optical fiber 710 illuminates homogenizing device722. Second light coupling system 716 illuminates second optical fiber718 which has core 720. Second optical fiber 718 combines with firstoptical fiber 710 to illuminate homogenizing device 722. Homogenizingdevice 722 illuminates projector 724. Rotating waveplate 704, PBS 706,and fold minor 714 form variable light splitter 730. Variable lightsplitter 730, first light coupling system 708, second light couplingsystem 716, first optical fiber 710 with core 712, and second opticalfiber 718 with core 720 form despeckling apparatus 700. Laser lightsource 702 may be a polarized, pulsed laser that has high enough peakpower to produce SRS in first optical fiber 710 and second optical fiber718. First light coupling system 708 and second light coupling system716 each may be one lens, a sequence of lenses, or other opticalcomponents designed to focus light into first core 712 and second core720 respectively. First optical fiber 710 and second optical fiber 718each may be an optical fiber with a core size and length selected toproduce the desired amount of SRS. First optical fiber 710 and secondoptical fiber 718 may be the same length or different lengths and mayhave the same core size or different core sizes. Additional elements maybe included to further guide or despeckle the light such as additionallenses, diffusers, vibrators, or optical fibers.

FIG. 8 shows a graph of power in the first optical fiber, color out ofthe first optical fiber, and color out of the second optical fiber vs.total power for an adjustable despeckling apparatus of the type shown inFIG. 7. The x-axis represents total average optical power in watts. Themathematical model used to derive FIG. 8 assumes no losses (such asscatter, absorption, or coupling) so the input power in each fiber isequal to the output power from each fiber. The total optical powerequals the sum of the power in the first fiber and the second fiber. Theleft y-axis represents power in percent, and the right y-axis representsGR color in percent. In the example of FIG. 8, the target color is theDCI green point (GR color=13.4%). By adjusting the variable lightsplitter, all points in FIG. 8 maintain the DCI green point for thecombined outputs of the two fibers. The two fibers are identical andeach has a core diameter and length selected such that they reach theDCI green point at 8 watts of average optical power. The cubicpolynomial fit described for FIG. 5 is used for the mathematicalsimulation of FIG. 8. First curve 800 represents the power in the firstfiber necessary to keep the combined total output of both fibers at theDCI green color point. Line 806 in FIG. 8 represents the DCI green colorpoint at a GR color of 13.4%. At 8 watts of total average power, 0%power into the first fiber and 100% power into the second fiber givesthe DCI green point because the second fiber is selected to give the DCIgreen point. As the total power is increased, the variable lightsplitter is adjusted so that more power is carried by the first fiber.The non-linear relationship between power and color (as shown in curve500 of FIG. 5) allows the combined output of both fibers to stay at theDCI green point while the total power is increased. At the maximumaverage power of 16 watts, the first fiber has 50% of the total power,the second fiber has 50% of the total power, and each fiber carries 8watts.

Second curve 802 in FIG. 8 represents the color of the output of thefirst fiber. Third curve 804 in FIG. 8 represents the color of theoutput of the second fiber. Third curve 804 reaches a maximum atapproximately 14 watts of total average power which is approximately 9watts of average power in the second fiber. Because 9 watts is largerthan the 8 watts necessary to reach DCI green in the second fiber, theGR color of light out of the second fiber is approximately 18% which ishigher than the 13.4% for DCI green. As the total average power isincreased to higher than 14 watts, the amount of light in the secondfiber is decreased. When 16 watts of total average power is reached,each fiber reaches 8 watts of average power. The example of FIG. 8 showsthat by adjusting the amount of power in each fiber, the overall colormay be held constant at DCI green even though the total average powervaries from 8 to 16 watts. Although not shown in FIG. 8, the despecklingis also held approximately constant over the same power range.

The previous example uses two fibers of equal length, but the lengthsmay be unequal in order to accomplish specific goals such as lowestpossible loss due to scattering along the fiber length, ease ofconstruction, or maximum coupling into the fibers. In an extreme case,only one fiber may be used, so that the second path does not passthrough a fiber. Instead of a variable light splitter based onpolarization, other types of variable light splitters may be used. Oneexample is a variable light splitter based on a wedged multilayercoating that moves to provide more or less reflection and transmissionas the substrate position varies. Mirror coatings patterned on glass canaccomplish the same effect by using a dense minor fill pattern on oneside of the substrate and a sparse minor fill pattern on the other sideof the substrate. The variable light splitter may be under softwarecontrol and feedback may be used to determine the adjustment of thevariable light splitter. The parameter used for feedback may be color,intensity, speckle contrast, or any other measurable characteristic oflight. A filter to transmit only the Raman-shifted light, only one Ramanpeaks, or specifically selected Raman peaks may be used with a photodetector. By comparing to the total amount of green light or comparingto the un-shifted green peak, the amount of despeckling may bedetermined. Other adjustment methods may be used instead of or inaddition to the two-fiber despeckler shown in FIG. 7. For example,variable optical attenuators may be incorporated into the fiber, thenumerical aperture of launch into the fiber may be varied, or fiber bendradius may be varied.

The example of FIG. 8 is a mathematical approximation which does notinclude second order effects such as loss and the actual spectrum ofSRS. Operational tests of an adjustable despeckler using two identicalfibers according to the diagram in FIG. 7 show that the actual range ofadjustability may be approximately 75% larger than the range shown inFIG. 8.

For a three-color laser projector, all three colors must have lowspeckle for the resultant full-color image to have low speckle. If thegreen light is formed from a doubled, pulsed laser and the red and bluelight are formed by an optical parametric amplifier (OPO) from the greenlight, the red and blue light may have naturally low speckle because ofthe broadening of the red and blue light from the OPO. A despecklingapparatus such as the one described in FIG. 7 may be used to despeckleonly the green light. A top view of such a system is shown in FIG. 9.First laser light source 926 illuminates first fold minor 928 whichilluminates light coupling system 932. Light coupling system 932illuminates second fold minor 930. Second fold mirror 930 illuminatesoptical fiber 934 which has core 936. Optical fiber 934 illuminateshomogenizing device 922. Second laser light source 902 illuminatesrotating waveplate 904. Rotating waveplate 904 changes the polarizationvector of the light so that it contains a desired amount of light ineach of two polarization states. Rotating waveplate 904 illuminates PBS906. PBS 906 divides the light into two beams. One beam with onepolarization state illuminates second light coupling system 908. Theother beam with the orthogonal polarization state reflects off thirdfold minor 914 and illuminates third light coupling system 916. Secondlight coupling system 908 illuminates second optical fiber 910 which hassecond core 912. Second optical fiber 910 combines with first opticalfiber 934 to illuminate homogenizing device 922. Third light couplingsystem 916 illuminates third optical fiber 918 which has core 920. Thirdoptical fiber 918 combines with first optical fiber 934 and secondoptical fiber 910 to illuminate homogenizing device 922. Third laserlight source 938 illuminates fourth fold minor 940 which illuminatesfourth light coupling system 944. Fourth light coupling system 944illuminates fifth fold mirror 942. Fifth fold mirror 942 illuminatesoptical fiber 946 which has core 948. Fourth optical fiber 946 combineswith first optical fiber 934, second optical fiber 910, and thirdoptical fiber 918 to illuminate homogenizing device 922. Homogenizingdevice 922 illuminates projector 924. Rotating waveplate 904, PBS 906,third fold minor 914, second light coupling system 908, third lightcoupling system 916, second optical fiber 910 with core 912, and thirdoptical fiber 918 with core 920 form despeckling apparatus 900. Firstlaser light source 926 may be a red laser, second laser light source 902may be a green laser, and third laser light source 938 may be a bluelaser. First laser light source 926 and third laser light source 938 maybe formed by an OPO which operates on light from second laser lightsource 902. Second laser light source 902 may be a pulsed laser that hashigh enough peak power to produce SRS in second optical fiber 910 andthird optical fiber 918. Additional elements may be included to furtherguide or despeckle the light such as additional lenses, diffusers,vibrators, or optical fibers.

FIG. 9 shows one color of light in each fiber. Alternatively, more thanone color can be combined into a single fiber. For example, red lightand blue light can both be carried by the same fiber, so that the totalnumber of fibers is reduced from four to three. Another possibility isto combine red light and one green light in one fiber and combine bluelight and the other green light in another fiber so that the totalnumber of fibers is reduced to two.

The despeckling apparatus may operate on light taken before, after, orboth before and after an OPO. The optimum location of the despecklingapparatus in the system may depend on various factors such as the amountof optical power available at each stage and the amount of despecklingdesired. FIG. 10 shows a block diagram of a three-color laser projectionsystem with despeckling of light taken after an OPO. First beam 1000enters OPO 1002. OPO 1002 generates second beam 1004, fourth beam 1010,and fifth beam 1012. Second beam 1004 enters despeckling apparatus 1006.Despeckling apparatus 1006 generates third beam 1008. First beam 1000,second beam 1004, and third beam 1008 may be green light. Fourth beam1010 may be red light, and fifth beam 1012 may be blue light.Despeckling apparatus 1006 may be a fixed despeckler or an adjustabledespeckler.

FIG. 11 shows a block diagram of a three-color laser projection systemwith despeckling of light taken before an OPO. First beam 1100 isdivided into second beam 1104 and third beam 1106 by splitter 1102.Third beam 1106 reflects from fold minor 1108 to create fourth beam1110. Fourth beam 1110 enters despeckling apparatus 1112. Despecklingapparatus 1112 generates fifth beam 1114. Second beam 1104 enters OPO1116. OPO 1116 generates sixth beam 1118 and seventh beam 1120. Firstbeam 1100, second beam 1104, third beam 1106, fourth beam 1110, andfifth beam 1114 may be green light. Sixth beam 1118 may be red light,and seventh beam 1120 may be blue light. Splitter 1102 may be a fixedsplitter or a variable splitter. Despeckling apparatus 1112 may be afixed despeckler or an adjustable despeckler.

FIG. 12 shows a block diagram of a three-color laser projection systemwith despeckling of light taken before and after an OPO. First beam 1200is divided into second beam 1204 and third beam 1206 by splitter 1202.Third beam 1206 reflects from fold mirror 1208 to create fourth beam1210. Fourth beam 1210 enters first despeckling apparatus 1212. Firstdespeckling apparatus 1212 generates fifth beam 1214. Second beam 1204enters OPO 1216. OPO 1216 generates sixth beam 1218, seventh beam 1224,and eighth beam 1226. Sixth beam 1218 enters second despecklingapparatus 1220. Second despeckling apparatus 1220 generates ninth beam1222. First beam 1200, second beam 1204, third beam 1206, fourth beam1210, fifth beam 1214, sixth beam 1218, and ninth beam 1222 may be greenlight. Seventh beam 1224 may be red light, and eighth beam 1226 may beblue light. Splitter 1202 may be a fixed splitter or a variablesplitter. First despeckling apparatus 1212 and second despecklingapparatus 1220 may be fixed despecklers or adjustable despecklers.

FIG. 13 shows a despeckling method that corresponds to the apparatusshown in FIG. 3. In step 1300, a laser beam is generated. In step 1302,the laser beam is focused into the core of an optical fiber. In step1304, SRS light is generated in the optical fiber. In step 1306, the SRSlight is used to form a projected digital image. Additional steps suchas homogenizing, mixing, splitting, recombining, and further despecklingmay also be included.

FIG. 14 shows an adjustable despeckling method that corresponds to theapparatus shown in FIG. 7. In step 1400, a first laser beam isgenerated. In step 1402, the first laser beam is split into second andthird laser beams. In step 1404, the second laser beam is focused intothe core of a first optical fiber. In step 1406, first SRS light isgenerated in the first optical fiber. In step 1410, the third laser beamis focused into the core of a second optical fiber. In step 1412, secondSRS light is generated in the second optical fiber. In step 1416, thefirst SRS light and the second SRS light is combined. In step 1420, thecombined SRS light is used to form a projected digital image. In step1422, the amount of light in the second and third beams is adjusted toachieve a desired primary color. Additional steps such as homogenizing,mixing, further splitting, further recombining, and further despecklingmay also be included.

Fibers used to generate SRS in a fiber-based despeckling apparatus maybe single mode fibers or multimode fibers. Single mode fibers generallyhave a core diameter less than 10 micrometers. Multimode fibersgenerally have a core diameter greater than 10 micrometers. Multimodefibers may typically have core sizes in the range of 20 to 400micrometers to generate the desired amount of SRS depending on theoptical power required. For very high powers, even larger core sizessuch as 1000 microns or 1500 microns may experience SRS. In general, ifthe power per cross-sectional area is high enough, SRS will occur. Alarger cross-sectional area will require a longer length of fiber, ifall other variables are held equal. The cladding of multimode fibers mayhave a diameter of 125 micrometers. The average optical power input intoa multimode fiber to generate SRS may be in the range of 1 to 200 watts.The average optical power input into a single mode fiber to generate SRSis generally smaller than the average optical power required to generateSRS in a multimode fiber. The length of the multimode fiber may be inthe range of 10 to 300 meters. For average optical power inputs in therange of 3 to 100 watts, the fiber may have a core size of 40 to 62.5micrometers and a length of 50 to 100 meters. The core material of theoptical fiber may be conventional fused silica or the core may be dopedwith materials such as germanium to increase the SRS effect or changethe wavelengths of the SRS peaks.

In order to generate SRS, a large amount of optical power must becoupled into an optical fiber with a limited core diameter. Forefficient and reliable coupling, specially built lenses, fibers, andalignment techniques may be necessary. 80 to 90% of the optical power ina free-space laser beam can usually be coupled into a multimode opticalfiber. Large-diameter end caps, metalized fibers, double clad fibers,antireflection coatings on fiber faces, gradient index lenses, hightemperature adhesives, and other methods are commercially available tocouple many tens of watts of average optical power into fibers with corediameters in the range of 30 to 50 micrometers. Photonic or “holey”fibers may be used to make larger diameters with maintainingapproximately the same Raman shifting effect. Average optical power inthe hundreds of watts can be coupled into fibers with core sizes in therange of 50 to 100 micrometers. The maximum amount of SRS, and thereforethe minimum amount of speckle, may be determined by the maximum powerthat can be reliably coupled into fibers.

Optical fibers experience scattering and absorption which cause loss ofoptical power. In the visible light region, the main loss is scattering.Conventional fused silica optical fiber has a loss of approximately 15dB per kilometer in the green. Specially manufactured fiber may begreen-optimized so that the loss is 10 dB per kilometer or less in thegreen. Loss in the blue tends to be higher than loss in the green. Lossin the red tends to be lower than loss in the green. Even with low-lossfiber, the length of fiber used for despeckling may be kept as short aspossible to reduce loss. Shorter fiber means smaller core diameter toreach the same amount of SRS and therefore the same amount ofdespeckling. Since the difficulty of coupling high power may place alimit on the amount of power that can be coupled into a small core,coupling may also limit the minimum length of the fiber.

Lasers used with a fiber-based despeckling apparatus may be pulsed inorder to reach the high peak powers required for SRS. The pulse width ofthe optical pulses may be in the range of 5 to 100 ns. Pulse frequenciesmay be in the range of 5 to 300 kHz. Peak powers may be in the range of1 to 1000 W. The peak power per area of core (PPPA) is a metric that canhelp predict the amount of SRS obtained. The PPPA may be in the range of1 to 5 kW per micrometer in order to produce adequate SRS fordespeckling. Pulsed lasers may be formed by active or passiveQ-switching or other methods that can reach high peak power. The modestructure of the pulsed laser may include many peaks closely spaced inwavelength. Other nonlinear effects in addition to SRS may be used toadd further despeckling. For example, self-phase modulation or four wavemixing may further broaden the spectrum to provide additionaldespeckling. Infrared light may be introduced to the fiber to increasethe nonlinear broadening effects.

The despeckling apparatus of FIG. 3 or adjustable despeckling apparatusof FIG. 7 may be used to generate more than one primary color. Forexample, red primary light may be generated from green light by SRS inan optical fiber to supply some or all of the red light required for afull-color projection display. Since the SRS light has low speckle,adding SRS light to other laser light may reduce the amount of specklein the combined light. Alternatively, if the starting laser is blue,some or all of the green primary light and red primary light may begenerated from blue light by SRS in an optical fiber. Filters may beemployed to remove unwanted SRS peaks. In the case of SRS from greenlight, the red light may be filtered out, or all peaks except the firstSRS peak may be filtered out. This filtering will reduce the colorchange for a given amount of despeckling, but comes at the expense ofefficiency if the filtered peaks are not used to help form the viewedimage. Filtering out all or part of the un-shifted peak may decrease thespeckle because the un-shifted peak typically has a narrower bandwidththan the shifted peaks.

The un-shifted peak after fiber despeckling is a narrow peak thatcontributes to the speckle of the light exciting the fiber. Thisunshifted peak may be filtered out from the spectrum (for example usinga dichroic filter) and sent into a second despeckling fiber to makefurther Raman-shifted peaks and thus reduce the intensity of theun-shifted peak while retaining high efficiency. Additional despecklingfibers may cascaded if desired as long as sufficient energy is availablein the un-shifted peak.

There are usually three primary colors in conventional full-colordisplay devices, but additional primary colors may also be generated tomake, for example, a four-color system or a five-color system. Bydividing the SRS light with beamsplitters, the peaks which fall intoeach color range can be combined together to form each desired primarycolor. A four-color system may consist of red, green, and blue primarieswith an additional yellow primary generated from green light by SRS inan optical fiber. Another four-color system may be formed by a redprimary, a blue primary, a green primary in the range of 490 to 520 nm,and another green primary in the range of 520 to 550 nm, where the greenprimary in the range of 520 to 550 nm is generated by SRS from the greenprimary in the range of 490 to 520 nm. A five-color system may have ared primary, a blue primary, a green primary in the range of 490 to 520nm, another green primary in the range of 520 to 550 nm, and a yellowprimary, where the green primary in the range of 520 to 550 nm and theyellow primary are generated by SRS from the green primary in the rangeof 490 to 520 nm.

3D projected images may be formed by using SRS light to generate some orall of the peaks in a six-primary 3D system. Wavelengths utilized for alaser-based six-primary 3D system may be approximately 440 and 450 nm,525 and 540 nm, and 620 and 640 nm in order to fit the colors into theblue, green, and red bands respectively and have sufficient spacingbetween the two sets to allow separation by filter glasses. Since thespacing of SRS peaks from a pure fused-silica core is 13.2 THz, thissets a spacing of approximately 9 nm in the blue, 13 nm in the green,and 17 nm in the red. Therefore, a second set of primary wavelengths at449 nm, 538 nm, and 637 nm can be formed from the first set of primarywavelengths at 440 nm, 525 nm, and 620 nm by utilizing the firstSRS-shifted peaks. The second set of primaries may be generated in threeseparate fibers, or all three may be generated in one fiber. Doping ofthe fiber core may be used to change the spacing or generate additionalpeaks.

Another method for creating a six-primary 3D system is to use theun-shifted (original) green peak plus the third SRS-shifted peak for onegreen channel and use the first SRS-shifted peak plus the secondSRS-shifted peak for the other green channel. Fourth, fifth, andadditional SRS-shifted peaks may also be combined with the un-shiftedand third SRS-shifted peaks. This method has the advantage of roughlybalancing the powers in the two channels. One eye will receive an imagewith more speckle than the other eye, but the brain can fuse a morespeckled image in one eye with a less speckled image in the other eye toform one image with a speckle level that averages the two images.Another advantage is that although the wavelengths of the two greenchannels are different, the color of the two channels will be moreclosely matched than when using two single peaks from adjacent greenchannels. Two red channels and two blue channels may be produced withdifferent temperatures in two OPOs which naturally despeckle the light.

Almost degenerate OPO operation can produce two wavelengths that areonly slightly separated. In the case of green light generation, twodifferent bands of green light are produced rather than red and bluebands. The two green wavelengths may be used for the two green primariesof a six-primary 3D system. If the OPO is tuned so that its two greenwavelengths are separated by the SRS shift spacing, SRS-shifted peaksfrom both original green wavelengths will line up at the samewavelengths. This method can be used to despeckle a system utilizing oneor more degenerate OPOs.

A different starting wavelength may used to increase the amount ofRaman-shifted light while still maintaining a fixed green point such asDCI green. For example, a laser that generates light at 515 nm may beused as the starting wavelength and more Raman-shifted light generatedto reach the DCI green point when compared to a starting wavelength of523.5 nm. The effect of starting at 515 nm is that the resultant lightat the same green point will have less speckle than light starting at523.5 nm.

When two separate green lasers, one starting at 523.5 nm and onestarting at 515 nm, are both fiber despeckled and then combined into onesystem, the resultant speckle will be even less than each systemseparately because of the increased spectral diversity. TheRaman-shifted peaks from these two lasers will interleave to make aresultant waveform with approximately twice as many peaks as each greenlaser would have with separate operation.

A separate blue boost may also be added from a narrow band laser at anydesired wavelength because speckle very hard to see in blue even withnarrow band light. The blue boost may be a diode-pumped solid-state(DPSS) or direct diode laser. The blue boost may form one of the bluepeaks in a six-primary 3D display. If blue boost is used, any OPOs inthe system may be tuned to produce primarily red or red only so as toincrease the red efficiency.

Peaks that are SRS-shifted from green to red may be added to the redlight from an OPO or may be used to supply all the red light if there isno OPO. In the case of six-primary 3D, one or more peaks shifted to redmay form or help form one or more of the red channels.

Instead of or in addition to fused silica, materials may be used thatadd, remove, or alter SRS peaks as desired. These additional materialsmay be dopants or may be bulk materials added at the beginning or theend of the optical fiber.

The cladding of the optical fiber keeps the peak power density high inthe fiber core by containing the light in a small volume. Instead of orin addition to cladding, various methods may be used to contain thelight such as minors, focusing optics, or multi-pass optics. Instead ofan optical fiber, larger diameter optics may used such as a bulk glassor crystal rod or rectangular parallelepiped. Multiple passes through acrystal or rod may be required to build sufficient intensity to generateSRS. Liquid waveguides may be used and may add flexibility when thediameter is increased.

Polarization-preserving fiber or other polarization-preserving opticalelements may be used to contain the light that generates SRS. Arectangular-cross-section integrating rod or rectangular-cross-sectionfiber are examples of polarization-preserving elements.Polarization-preserving fibers may include core asymmetry or multiplecores that guide polarized light in such a way as to maintainpolarization.

In a typical projection system, there is a trade-off between brightness,contrast ratio, uniformity, and speckle. High illumination f# tends toproduce high brightness and high contrast ratio, but also tends to givelow uniformity and more speckle. Low illumination f# tends to producehigh uniformity and low speckle, but also tends to give low brightnessand low contrast ratio. By using spectral broadening to reduce speckle,the f# of the illumination system can be raised to help increasebrightness and contrast ratio while keeping low speckle. Additionalchanges may be required to make high uniformity at high f#, such as alonger integrating rod, or other homogenization techniques which areknown and used in projection illumination assemblies.

If two OPOs are used together, the OPOs may be adjusted to slightlydifferent temperatures so that the resultant wavelengths are different.Although the net wavelength can still achieve the target color, thebandwidth is increased to be the sum of the bandwidths of the individualOPOs. Increased despeckling will result from the increased bandwidth.The bands produced by each OPO may be adjacent, or may be separated by agap. In the case of red and blue generation, both red and blue will bewidened when using this technique. For systems with three primarycolors, there may be two closely-spaced red peaks, four or more greenpeaks, and two closely-spaced blue peaks. For systems with six primarycolors, there may be three or more red peaks with two or more of the redpeaks being closely spaced, four or more green peaks, and three or moreblue peaks with two or more of the blue peaks being closely spaced.Instead of OPOs, other laser technologies may be used that can generatethe required multiple wavelengths.

Screen vibration or shaking is a well-know method of reducing speckle.The amount of screen vibration necessary to reduce speckle to atolerable level depends on a variety of factors including the spectraldiversity of the laser light impinging on the screen. By using Raman tobroaden the spectrum of light, the required screen vibration can bedramatically reduced even for silver screens or high-gain white screensthat are commonly used for polarized 3D or very large theaters. Thesespecialized screens typically show more speckle than low-gain screens.When using Raman despeckling, screen vibration may be reduced to a levelon the order of a millimeter or even a fraction of a millimeter, so thatscreen vibration becomes practical and easily applied even in the caseof large cinema screens.

Other implementations are also within the scope of the following claims.

1. An optical apparatus comprising: A first optical fiber; wherein astimulated Raman scattering in the first optical fiber reduces a firstspeckle characteristic of a first light output from the first opticalfiber.
 2. The apparatus of claim 1 wherein the first optical fiber has afirst core diameter and a first length and the first core diameter andthe first length are selected to achieve a first desired amount ofreduction in the first speckle characteristic.
 3. The apparatus of claim1 wherein the first optical fiber is a multimode fiber.
 4. The apparatusof claim 1 wherein the first core diameter and the first length areselected to achieve a desired color of the light output.
 5. Theapparatus of claim 4 wherein the desired color is a primary color. 6.The apparatus of claim 5 wherein the primary color is a green color. 7.The apparatus of claim 6 further comprising: an optical parametricamplifier; wherein the optical parametric amplifier generates a blueprimary color and a red primary color.
 8. The apparatus of claim 6wherein a portion of the stimulated Raman scattering forms a redstimulated Raman scattering light.
 9. The apparatus of claim 8 furthercomprising: a red light source; wherein the red light source is combinedwith the red stimulated Raman scattering light.
 10. The apparatus ofclaim 6 wherein a portion of the stimulated Raman scattering forms ayellow stimulated Raman scattering light.
 11. The apparatus of claim 10wherein the yellow stimulated Raman scattering light is used to form aprojected digital image with four primary colors.
 12. The apparatus ofclaim 8 wherein the green color has a color chromaticity ofu′=0.099+/−0.01 and v′=0.578+/−0.01.
 13. The apparatus of claim 1further comprising: a laser light source; and a digital projector;wherein the laser light source illuminates the first optical fiber, andthe first optical fiber illuminates the digital projector.
 14. Theapparatus of claim 13 wherein the laser light source comprises: a pulsedlaser; and a second harmonic generator.
 15. The apparatus of claim 14wherein the laser light source has an output at 523.5 nm.
 16. Theapparatus of claim 14 wherein the laser light source has an output at515 nm.
 17. The apparatus of claim 14 wherein the laser light source hasan output at 532 nm.
 18. The apparatus of claim 1 further comprising: alight coupling system; and a homogenizing device; wherein the laserlight source illuminates the light coupling system, the light couplingsystem illuminates the first optical fiber, the first optical fiberilluminates the homogenizing device, and the homogenizing deviceilluminates the digital projector.
 19. The apparatus of claim 1 furthercomprising: a second optical fiber; and a variable light splitter;wherein the variable light splitter illuminates the first optical fiberand the second optical fiber, and a stimulated Raman scattering in thesecond optical fiber reduces a second speckle characteristic of a secondlight output from the second optical fiber.
 20. The apparatus of claim19 wherein the variable light splitter comprises: a rotating waveplate;and a polarizing beam splitter.
 21. The apparatus of claim 19 whereinthe second optical fiber has a second core diameter and a second lengthand the second core diameter and the second length are selected toachieve a second desired amount of reduction in the second specklecharacteristic.
 22. The apparatus of claim 19 further comprising: alaser light source; and a digital projector; wherein the laser lightsource illuminates the variable light splitter, and the first opticalfiber and the second optical fiber illuminate the digital projector. 23.The apparatus of claim 19 wherein the variable light splitter isadjusted to achieve a desired color for a combined light output of thefirst optical fiber and the second optical fiber.
 24. A laser projectorcomprising: a blue laser light source; an optical fiber; and a spatiallight modulator; wherein the blue laser light source illuminates theoptical fiber, the optical fiber illuminates the spatial lightmodulator, and a stimulated Raman scattering in the optical fiberproduces a despeckled green light and a despeckled red light.
 25. Amethod of despeckling comprising: generating a laser beam; focusing thelaser beam into an optical fiber; generating stimulated Raman scatteringlight in the optical fiber; and using the stimulated Raman scatteringlight to form a projected digital image.
 26. A method of despecklingcomprising: generating a first laser beam; splitting the first laserbeam into second and third laser beams; focusing the second laser beaminto a first optical fiber; generating a first stimulated Ramanscattering light in the first optical fiber; focusing the third laserbeam into a second optical fiber; generating a second stimulated Ramanscattering light in the second optical fiber; combining the firststimulated Raman scattering light and the second stimulated Ramanscattering light to form a combined stimulated Raman scattering light;using the combined stimulated Raman scattering light to form a projecteddigital image; and adjusting the amount of light in the second laserbeam and the third laser beam to achieve a desired color for a primarycolor in the projected digital image.